Transponder Power Supply, a Transponder and a Method for Providing a Transponder Power Supply Current

ABSTRACT

A transponder power supply for providing a supply current based on an antenna input signal. The transponder power supply comprises an emergency circuit comprising a charging circuit, an emergency capacitor, and an output stage. The charging circuit is configured to charge the emergency capacitor based on the antenna input signal to a maximum voltage which is higher than a voltage of the antenna input signal. The output stage is configured to provide a contribution to the supply current using a charge of the emergency capacitor.

FIELD OF THE INVENTION

Embodiments of the present invention relate to transponder powersupplies, a transponder, and methods for providing a transponder powersupply.

BACKGROUND

In case of proximity contactless chip cards (for example radio frequencyidentification (RFID)—transponders) energy as well as information istransferred over an inductively coupled antenna interface. Informationfrom a so-called reader (PCD—proximity coupling device) to a chip cardor a transponder (PICC—proximity integrated circuit card) is typicallytransferred by amplitude modulations and by load modulations in theother direction from the PICC to the PCD. The communication for the13.56 MHz ISO 14443 standard is defined by A/B/C/standards.

By using the type A Standard, amplitude modulation with a modulationindex of up to 100% is used. A single bit is coded as a field pause.During the pause, the field emitted by the PCD antenna is reduced by themodulation index. With a modulation index of 100%, the field emitted bythe PCD antenna is turned off during a type A pause. A 100% type A pausecan last several microseconds. As the emitted field of the PCD antennais also used to transfer energy to the PICC, the PICC is not suppliedwith energy during this time interval.

Therefore an energy reservoir (for example, a capacitor) is sometimesadded to the PICC, to provide the amount of energy consumed by the PICCcircuitry during the abovementioned type A gap or type A pause or fieldpause.

As the capacitor which is used, or even required in some cases, forsurviving the longest type A gap possible (which is defined in the ISO14443 Standard) is quite large, this capacitor significantly increasesthe area of the analog frontend of the contactless chip card or RFIDtransponder.

There is a desire in the art for providing a concept, which allows for amore efficient implementation of a power supply for transponders.

SUMMARY

Embodiments of the present invention provide a transponder power supplyfor providing a supply current based on an antenna input signal. Thetransponder power supply comprises an emergency circuit. The emergencycircuit comprises a charging circuit, an emergency capacitor and anoutput stage. The charging circuit is configured to charge the emergencycapacitor based on the antenna input signal to a maximum voltage, whichis higher than a voltage of the antenna input signal. The output stageis configured to provide a contribution to the supply current using acharge of the emergency capacitor.

Further embodiments of the present invention provide a transponder powersupply for providing a supply current based on an antenna input signal.The transponder power supply comprises an emergency circuit. Theemergency circuit comprises a charging circuit, an emergency capacitorand an output stage. The charging circuit is configured to charge theemergency capacitor based on the antenna input signal. The output stagecomprises a step down converter circuit configured to provide acontribution to the supply current using a charge of the emergencycapacitor.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in the following,referring to the accompanying figures, wherein:

FIG. 1 shows a block diagram of a transponder power supply according toan embodiment of the present invention;

FIG. 2 shows a block diagram of a transponder power supply according toa further embodiment of the present invention;

FIG. 3 shows a block diagram of a transponder power supply according toa further embodiment of the present invention;

FIG. 4 shows a block diagram of an emergency circuit, which may beemployed in the transponder power supplies according to FIG. 1 to FIG.3;

FIG. 5 shows an example of a timing diagram of a singlecharge/discharge-cycle using the emergency circuit according to FIG. 4;

FIG. 6 shows a block diagram of a transponder according to a furtherembodiment of the present invention;

FIG. 7 shows a flow diagram of a method according to a furtherembodiment of the present invention; and

FIG. 8 shows a flow diagram of a method according to a furtherembodiment of the present invention.

DETAILED DESCRIPTION

Before embodiments of the present invention are explained in greaterdetail in the following taking reference to the figures, it is to bepointed out that the same or functionally equal elements are providedwith the same reference numbers, and that a repeated description ofthese elements will be omitted. Hence, the description of elementsprovided with the same reference number is mutually interchangeableand/or applicable in the various embodiments.

FIG. 1 shows a block diagram of a transponder (e.g. contactless chipcard) power supply 100 according to an embodiment of the presentinvention.

The transponder power supply 100 for providing a supply current 101based on an antenna input signal 102 comprises an emergency circuit 103.

The emergency circuit may, in the following, also be called EMC moduleor EMC block.

The emergency circuit 103 comprises a charging circuit 104, an emergencycapacitor 105, and an output stage 106. The charging circuit 104 isconfigured to charge the emergency capacitor 105 based on the antennainput signal 102 to a maximum voltage, which is higher than a voltage ofthe antenna input signal 102. For example, the charging circuit 104 maybe configured to charge the emergency capacitor 105 to a voltage, whichis higher than a peak voltage of the antenna input signal 102. Theoutput stage 106 is configured to provide a contribution 107 to thesupply current 101, using a charge 108 of the emergency capacitor 105.

A voltage of the antenna input signal 102 is in the following alsocalled antenna voltage.

Within the present application the term transponder may designate adevice comprising an receiver circuit or a transceiver circuit coupledto an antenna, which is able to generate a supply voltage and/or asupply current for the receiver circuit or the transceiver circuit basedon an antenna input signal. A transponder may for example be anRFID-transponder, e.g. a wireless contactless chip card, which isconfigured to communicate with a reader through a wireless communicationlink and to generate a supply voltage and/or supply current based on a(magnetic) field generated by the reader. The (magnetic) field generatedby the reader can also be used to transfer data. A typical transpondermay not comprise a battery. However, there may be use cases, in which atransponder comprises a battery and in which the above mentionedemergency circuit 103 may be used in a case of an empty battery orsimilar cases.

It is an idea of embodiments of the present invention that an efficientimplementation of a transponder power supply can be achieved if thevoltage at the emergency capacitor 105 can be increased to be higherthan the voltage of the antenna input signal 102. It has been found thatby increasing the voltage at the emergency capacitor 105 above the(maximum) voltage of the antenna input signal 102, the capacitance andas a result the size of the emergency capacitor 105 can be reduced(compared to the case in which the maximum voltage at the emergencycapacitor is equal to a maximum voltage of an antenna input signal) bykeeping the amount of charge stored in the emergency capacitor 105, orin other words, while avoiding a reduction of the amount of energystored in the emergency capacitor 105. Embodiments of the presentinvention therefore enable an optimal usage of the emergency capacitor105 by charging the emergency capacitor 105 to its maximum possiblevoltage, and therefore, bringing more charge into the same capacitorthan in the case where the capacitor is only charged to a voltage belowits maximum voltage (e.g. to a voltage equal to the maximum voltage ofan antenna input signal).

In typical conventional transponder power supplies, a voltage of thetransponder input signal 102 may be limited to 5 V. However it has beenfound that medium voltage capacitors may be typically charged up toabout 7 V. Therefore the charging circuit 104 may charge the emergencycapacitor 105 to 7 V, even if the maximum voltage of the antenna inputsignal 102 is below 7 V. Furthermore, the voltage of the antenna inputsignal 102 significantly decreases if the distance between the PCD (thetransponder) and the PICC (the transponder reader) increases (so calledweak field supply case). In other words, the voltage of the antennainput signal 102 is strongly dependent on the distance betweentransponder and transponder reader.

The voltage of the antenna input signal 102 may therefore be even belowthe above mentioned 5 V, but by using the transponder power supply 100shown in FIG. 1, the emergency capacitor 105 may also be charged to itsmaximum voltage (e.g. to 7V).

It's an advantage of some embodiments of the present invention that bycharging the emergency capacitor 105 to a higher voltage than theantenna input signal 102, more charge may be stored in the samecapacitor than in concepts where the voltage of the emergency capacitorequals the voltage of the antenna input signal and/or a size of theemergency capacitor 105 may be smaller than a size of emergencycapacitors in circuits based on concepts wherein the voltage of thecapacitor equals or is below the voltage of the antenna input signal.

Embodiments of the present invention therefore create a concept whichallows for a smaller emergency capacitor and/or a longer abovementionedtype A gap or field pause within transponders, such as contactless chipcards.

The contribution 107 to the supply current 101, which is provided by theoutput stage 106, may be the main part, or even the exclusive part, ofthe supply current 101 consumed by a connected digital chip load, forexample, during a field pause, in which a regular power supplyconfigured to deliver another contribution to the supply current 101cannot deliver sufficient current to the digital chip load.

FIG. 2 shows a transponder power supply 200 according to a furtherembodiment of the present invention. The transponder power supply 200for providing a power supply current 101 based on an antenna inputsignal 102, comprises an emergency circuit 203. The emergency circuit203 comprises a charging circuit 204, an emergency capacitor 105, and anoutput stage 206. The charging circuit 204 is configured to charge theemergency capacitor 105 based on the antenna input signal 102. Theoutput stage 206 comprises a step down converter circuit 207 configuredto provide a contribution 107 to the supply current 101 using a charge108 of the emergency capacitor 105.

It is another idea of some embodiments of the present invention that amore efficient concept for a transponder power supply can be provided ifthe contribution 107 to the supply current 101 is provided using thestep down converter circuit 107.

It is an effect of some embodiments of the present invention that avoltage at the emergency capacitor 105 can be converted to a powersupply voltage, which is lower than the voltage at the emergencycapacitor 105, using the step down converter circuit 207. It has beenfound that, in comparison to a linear approach, the energy transfer fromthe medium voltage domain (for example, around 5 V or up to 7V) to a lowvoltage domain (for example, 1.4 V) is therefore more efficient and lessenergy is wasted during the charge transfer. It has been found that in acontinuous linear discharge approach the energy lost by the chargetransfer itself is typically proportional to the voltage differencebetween the voltage at the emergency capacitor and the required supplyvoltage. It has been found that due to this fact, the continuous lineardischarge approach is very inefficient compared to the concept shown inFIG. 2, in which the step down converter circuit 207 is used to providethe contribution 107 to the power supply current 101.

The step down converter circuit 207, may for example, be a switchedcapacitor step down converter circuit configured to provide a step downoutput voltage (the power supply voltage) based on a switching between aserial connection of at least two step down capacitors and a parallelconnection of the at least two step down capacitors.

By transferring charge from the medium voltage level at the emergencycapacitor 105 to the low voltage level at an output terminal of theoutput stage 206, which may also be an output terminal of the emergencycircuit 203, by using the step down converter circuit 207, less energyis wasted, and therefore the charge in the emergency capacitor 105 maylast longer and therefore a field pause may be longer than in conceptsin which no step down converter circuit is used. Furthermore, due to thefact that less energy is wasted during the charge transfer, theemergency capacitor 105 may be smaller and may therefore have a smallercapacitance and a smaller maximum charge. But due to the fact that lessenergy is wasted during the charge transfer, the maximum field pause maystill be the same than in concepts in which no step down convertercircuit is used.

Embodiments of the present invention therefore provide a more efficientimplementation of a transponder power supply comprising a smaller size,as a result of the reduced size of the capacitor, and/or enabling thepossibility of a longer field pause.

According to further embodiments, the charging circuit 204 shown in FIG.2 may be replaced with the charging circuit 104 shown in FIG. 1, whichis configured to charge the emergency capacitor 105 based on the antennainput signal 102 to a maximum voltage which is higher than the voltageof the antenna input signal 102.

According to further embodiments, the output stage 106 shown in FIG. 1may be replaced with the output stage 206 shown in FIG. 2, comprisingthe step down converter circuit 207.

FIG. 3 shows a block diagram of a transponder power supply 300 accordingto a further embodiment of the present invention with a connectedantenna 308 and external tuning capacitor 309. The transponder powersupply 300 for providing a power supply current 101 (a current I_(VDD))based on an antenna input signal 102 (comprising a voltage VLA/LB and acurrent I_(LA/LB)) comprises an emergency circuit 303 and a regularpower supply circuit 310. The emergency circuit 303 and the regularpower supply circuit 310 are coupled in parallel in between an antennainterface terminal 311 of the transponder power supply 300 and a commonVDD node 312 of the transponder power supply 300. The antenna 308 andthe external tuning capacitor 309 are coupled to the antenna interfaceterminal 311. Furthermore, the transponder power supply 300 comprises anantenna shunt regulator 313 and a demodulator 314 coupled to the antennainterface terminal 311.

The transponder power supply 300 comprises a VDD shunt regulator 315 anda VDD capacitor 316 coupled to the common VDD node 312.

The emergency circuit 303 comprises a charging circuit 304, an emergencycapacitor 105, an output stage 306, and a digital control module 317.The charging circuit 304 is configured to charge the emergency capacitor105 based on the antenna input signal 102. The charging circuit 304 may,for example, be equal to the charging circuit 104 of the transpondersupply 100, which means that the charging circuit 304 may be configuredto charge the emergency capacitor 105 based on the antenna input signal102 to a maximum voltage, which is higher than the voltage of theantenna input signal 102. The output stage 306 is configured to providea contribution 307 (a current I_(EMC)) to the supply current 101 using acharge of the emergency capacitor 105. The output stage 306 may, forexample, be equal to the output stage 206 of the transponder powersupply 200 from FIG. 2, i.e., the output stage 206 may comprise a stepdown converter circuit 207 configured to provide the contribution 307 tothe supply current 101. The emergency circuit 303 may, therefore, be acombination of the emergency circuit 103 and the emergency circuit 203,or may be equal to one of the emergency circuits 103 or 203.

The regular power supply circuit 310 comprises a main rectifier 318, aVDD RF capacitor 319, and a current source 320.

In the following, it is assumed that the emergency circuit 303 is acombination of the emergency circuit 103 from FIG. 1 and the emergencycircuit 203 from FIG. 2. I.e. the charging circuit 304 is configured tocharge the emergency capacitor 105, based on the antenna input signal102, to a maximum voltage which is higher than a voltage of the antennainput signal 102, and the output stage 306 comprises a step downconverter circuit 207 (not shown in FIG. 3) configured to provide thecontribution 307 to the supply current 101, using the charge of theemergency capacitor 105. Using the combination of the emergency circuit103 and the emergency circuit 203 leads to the emergency circuit 303 ofthe transponder supply 300 comprising a maximum efficiency.Nevertheless, there may be cases which arise wherein it is advantageousto only employ the emergency circuit 103 or the emergency circuit 203.

In the following, the working principle of the transponder power supply300 will be explained in detail.

During normal operation, this means a magnetic field is present at theantenna 308 and the voltage (VLA/LB) of the antenna input signal 102 isabove zero, the current source 320, providing a supply voltage VDD atthe common VDD node 312, is supplied via the main rectifier 318. In atypical embodiment, the maximum voltage of the antenna input signal 102is 5 V and controlled (or limited) by the antenna shunt regulator 313.The emergency capacitor 105 is charged up by the charging circuit 304 tothe nominal voltage of a medium voltage capacitor independently from thepresent antenna voltage. In other words, the emergency capacitor 105 ischarged up to a voltage higher than the voltage of the antenna inputsignal 102, which is also an input voltage of the charging circuit 304.As can be seen from FIG. 3, the charging circuit 304 provides a chargecurrent I_(charge) to the emergency capacitor 105. Due to this propertyof charging the emergency capacitor 105 to a higher voltage than thevoltage of the antenna input signal 102, the charging circuit 304 mayalso be called a charge pump circuit. The charging circuit 304 may becontrolled by the digital control module 317 in a manner in which thedistortion of the antenna input signal 102 and its voltage (especiallyduring data transmission) is at a minimum, using a charge control signal321.

As in this phase (i.e., the magnetic field is present at the antenna 308and the voltage (VLA/LB) of the antenna input signal 102 is above zero)the supply voltage VDD and the supply current 101 are fully supplied bythe current source 320, the output stage 306 of the emergency circuit303 may be turned off by the digital control module 317 using adischarge enable signal 322. The output stage 306 is therefore notconsuming any current (or only an insignificant amount of current, e.g.leakage current) in the phase, in which the supply voltage VDD isdirectly supplied on the basis of the magnetic field coupled into theantenna 308.

When the antenna voltage VLA/LB (the voltage of the antenna input signal102) becomes very low (for example, during a type A field gap), the mainrectifier 318 cannot deliver any current to the current source 320, andas still some supply current 101 is consumed by a circuit coupled to theVDD node 312, the supply current 101 or at least a substantial portionof the supply current 101 needs to be supplied by the emergency circuit303 sourcing the contribution 307 (the current I_(EMC)) to the VDD node312. In other words, during a type A gap, the antenna voltage VLA/LBbecomes very low and the main rectifier 318 turns off (for example forseveral microseconds). The field pause or the type A gap is detected bythe demodulator 314 and this information is provided to the digitalcontrol module 317 by a pause signal 323. The load current (the supplycurrent 101 or the current I_(VDD)) still consumed at the VDD node 312is compensated by the emergency circuit current I_(EMC) (by thecontribution 307 to the supply current 101). In other words, during atype A gap or field pause, the charging circuit 304 is turned off andthe output stage 306 (comprising the step down converter circuit 207) isactivated by the digital control module 317, sourcing the currentI_(EMC) to the VDD node 312, which is equivalent or insignificantlydifferently from the VDD load current I_(VDD) (the supply current 101).Due to this fact, no (or only an insignificantly low amount of) currentis wasted in the VDD shunt regulator 315.

The end of the type A gap or field pause is again detected by thedemodulator 314 and in response to this the output stage 306 is turnedoff and the charging circuit 304 is turned on to recharge the emergencycapacitor 105.

According to some embodiments of the present invention, the digitalcontrol module 317 may be configured to provide a charge pump adjustsignal to the charging circuit 304. The charging circuit 304 maytherefore be configured to vary an amount of charge transferred within agiven time unit from the antenna input signal 102 to the emergencycapacitor 105. After detection of an end of the type A gap or the fieldpause, the charging circuit 304 may therefore be softly turned on, suchthat the antenna input signal 102 is disturbed as little as possible.The charge pump adjust signal may, for example, be part of the chargecontrol signal 321.

To summarize, the transponder power supply 300 comprises the emergencycircuit 303 and the regular power circuit 310, coupled in parallel tothe emergency circuit 303. The regular power circuit 310 is configuredto provide another contribution 324 (the current I_(VDDCRS)) to thesupply current 101 at the common VDD node 312 if the voltage of theantenna input signal is above a given input threshold voltage. Theemergency circuit 303 is configured to selectively provide itscontribution 307 to the supply current 101 at the common VDD node 312 ifthe voltage of the antenna input signal 102 is equal to, or below, thegiven input threshold voltage.

It should be mentioned that during a type A gap or a field pause, thevoltage of the antenna input signal 102 is equal to or below thementioned given input threshold voltage.

It should also be mentioned that at the moment when the voltage of theantenna input signal 102 falls below the given input threshold voltageand the output stage 306 becomes enabled to deliver its contribution 307to the supply current 101, the regular power circuit 310 may stilldeliver a small amount of current and therefore may still deliver itscontribution 324 to the supply current 101. The small amount of currentmay, for example, be a discharge current of the VDDRF capacitor 319.After discharging the VDDRF capacitor 319, the current I_(VDDRF) maybecome zero and the contribution 324 of the regular power circuit 310may also become zero. For a maximum efficiency of the transponder powersupply 300, the output stage 306 may adapt its contribution 307 (thecurrent I_(EMC)) such that it is equivalent to the supply current 101consumed at the VDD node 312, such that no current is wasted in the VDDshunt regulator 315, or in other words, a current I_(SHUNT) floating outof the VDD node 312 into the VDD shunt regulator 315 may become zero.

Some advantages of the transponder power supply 300 shall be mentionedin the following. Due to the fact of using the charging circuit 304,which is configured to charge the emergency capacitor 105 to a maximumvoltage which is above the voltage of the antenna input signal 102, themaximum voltage at the emergency capacitor 105 is, at least within agiven range of the voltage of the antenna input signal 102, independentfrom the voltage of the antenna input signal 102. Therefore the fullvoltage range of the emergency capacitor 105 may be used, allowing asignificant area reduction, as the capacitance and the size (the area itconsumes) of the emergency capacitor can be reduced by at least a factorof 1.5 or 1.7, or 2. Additionally, the use of the step down convertorcircuit 107 within the output stage 306 reduces or even dramaticallyreduces the energy loss caused by the charge transfer from the emergencycapacitor 105 to the VDD node 312, again allowing an area reduction ofthe emergency capacitor 105, as less charge is wasted during thedischarge cycle. Furthermore, the current I_(EMC) (the

FIG. 4 shows, in an example, the emergency circuit 303 from FIG. 3 in amore detailed version. The emergency circuit 303 comprises the chargingcircuit 304, the output stage 306, and an emergency capacitor module401. The emergency capacitor 105 of the emergency circuit 303 iscomprised in the emergency capacitor module 401. The charging circuit304 comprises a first charge pump 402 coupled to an antenna port 403 forthe antenna input signal 102. As can be seen from FIG. 4, the antennainput signal 102 may be a fully differential signal, this means theantenna input signal 102 may comprise a first differential input signal102 a (LA) and a second differential input signal 102 b (LB). Theantenna port 403 may be directly connected to the antenna interfaceterminal 311 of the transponder power supply 300. The first charge pump402 is configured to provide a first charge current 404 to the emergencycapacitor 105, based on the antenna input signal 102. In other words,the first charge pump 402 is configured to transfer charge from theantenna input signal 102 to the emergency capacitor 105. The firstcharge pump 402 may also be called LA/LB charge pump 402.

As mentioned before, the charging circuit 304 may be configured to varythe amount of charge transferred from the antenna input signal 102 tothe emergency capacitor 105. In the embodiment shown, the first chargepump 402 may perform this function, meaning that the first charge pump402 may be configured to vary an amount of charge transferred from theantenna input signal 102 to the emergency capacitor 105 in dependence ona charge pump adjust signal 405 (which may be part of the charge controlsignal 321). The transponder power supply 300 may therefore comprise acharge pump adjuster 406 (for example, comprised in the digital controlmodule 317) configured to provide the charge pump adjust signal 405 independence on a voltage of the antenna input signal 102. The charge pumpadjuster 406 may provide the charge pump adjust signal 405, such that afirst amount of charge transferred within a given time unit in a firststate in which an amplitude of the voltage of the antenna input signal102 is comparatively small, is smaller than a second amount of chargetransferred within the given time unit in a second state, in which theamplitude of the voltage of the antenna input signal 102 iscomparatively larger. In other words, the charge pump adjuster 406 maybe configured to adjust the first charge pump 402 such that the strengthof the first charge pump 402, and in the end the current sunk from theantenna port 403 during the charge cycle, is adjusted by the charge pumpadjust signal 405 in a manner that the distortion of the antenna voltageand of the antenna input signal 102 is minimized under all operatingconditions. This is advantageous because the first charge pump 402 issupplied by the first differential antenna signal 102 a and the seconddifferential antenna input signal 102 b, i.e. by the antenna inputsignal 102.

In addition to the first charge pump 402, the charging circuit 304 maycomprise a second charge pump 407 configured to provide a second chargecurrent 408 to the emergency capacitor 105 based on a supply voltage VDDat the common VDD node 312, or in other words, by transferring chargefrom the common VDD node 312. In other words, the second charge pump 407is supplied from the VDD node 312 (based on the supply voltage VDD) andthe first charge pump 402 is supplied from the antenna port 403 (basedon the antenna voltage). An output 409 of the first charge pump 402providing the first charge current 404 and an output 410 of the secondcharge pump 407 providing the second charge current 408 are coupled tothe emergency capacitor 105, such that an overall charge current 411 ofthe emergency capacitor 105 is a sum of the first charge current 404 andthe second charge current 408.

The first charge pump 402 and the second charge pump 407 may beconfigured such that a maximum charge current provided by the firstcharge pump 402 is larger, at least by a factor of 2, or of 5, or of 10,or of 20, or even of 50, than a maximum charge current provided by thesecond charge pump 407. According to further embodiments other valuesfor the factor are also possible. The first charge pump 402 maytherefore be implemented for quickly recharging the emergency capacitor105 from a low voltage level (for example, after a type A gap or a fieldpause). The second charge pump 407 may be implemented to compensatecurrent consumption and leakage current in the emergency capacitormodule 401. According to some embodiments the second charge pump 407 mayonly be used for compensating leakage at the emergency capacitor 105.

As mentioned before, the digital control module 317 (not shown in FIG.4), may activate the recharging circuit 304 if the voltage of theantenna input signal 102 is above a given threshold input voltage andmay furthermore deactivate the output stage 306, if the voltage of theantenna input signal 102 is above the given threshold input voltage. Thedigital control module 317 may therefore comprise a power controlcircuit configured to activate and deactivate the charging circuit 304,in dependence on the voltage of the antenna input signal 102. The firstcharge pump 402 and/or the second charge pump 407 may, therefore, beconfigured to selectively provide the first charge current 404 and/orthe second charge current 408, only if the voltage of the antenna inputsignal 102 is above the given threshold input voltage.

As mentioned before, the second charge pump 407 may be implemented tocompensate current consumption and leakage current in the emergencycapacitor module 401. The power control circuit may therefore further beconfigured to activate and deactivate the second charge pump independence of the voltage at the emergency capacitor 105. The secondcharge pump 407 may therefore be configured to provide the second chargecurrent 408 until the emergency capacitor 105 reaches its maximumvoltage, to interrupt providing the second charge current 408 until thevoltage at the emergency capacitor 105 falls below a given leakagecurrent emergency capacitor threshold voltage and to resume providingthe second charge current 408.

The power control circuit may therefore be configured to activate both,the first charge pump 402 and the second charge pump 407, after anincrease of the antenna input signal from a voltage below the giveninput threshold voltage to a voltage above the given threshold voltagein order to charge the emergency capacitor 105 to the maximum voltage,to subsequently deactivate both, the first charge pump 402 and thesecond charge pump 407, and to keep the voltage at the emergencycapacitor 105 within a predetermined range using the second charge pump407 (or using the second charge pump 407 only), while the voltage of theantenna input signal 102 is above the given input threshold voltage. Thepredetermined range may, for example, be a range from the maximumvoltage of the emergency capacitor 105 down to 90% of the maximumvoltage, down to 80% of the maximum voltage, down to 75% of the maximumvoltage, or even down to 50% of the maximum voltage of the emergencycapacitor 105.

According to further embodiments, the control circuit may be configuredto activate only the first charge pump 402 after (or in response to) theincrease of the antenna input signal voltage from below the given inputthreshold voltage to a voltage above the given input threshold voltageuntil the maximum voltage of the emergency capacitor 105 has beenreached, and to use the second charge pump 407 only to keep the voltageat the emergency capacitor 105 within the predetermined range.

In other words, the charging circuit 304 implements a small (VDDsupplied) second charge pump 407 and a large (VLA/LB supplied) firstcharge pump 402. The first charge pump 402 is turned on after thedischarge phase for quickly recharging the emergency capacitor 105 froma low voltage level. For a faster recharging the second charge pump 407may also be turned on, e.g. after the supply voltage VDD has beenstabilized. The strength of the first charge pump 407 and in the end thecurrent sunk from the antenna port 402 during the charge cycle may beadjusted by the charge pump adjust signal 405 in a manner that thedistortion of the antenna voltage is minimized under all operatingconditions. In addition to the first charge pump 402, the second chargepump 407 is active as long as the voltage at the emergency capacitor 105does not reach its target value (the maximum voltage). The second chargepump 407 may be implemented to compensate current consumption andleakage current in the emergency capacitor module 401.

As can be seen from FIG. 4 may the emergency capacitor module 401comprise a voltage divider 412, used to sense the voltage of theemergency capacitor 105, an emergency capacitor shunt (EMC shunt) 413used to clamp the voltage at the emergency capacitor 105 and a firstcomparator 414 (which may also be part of the abovementioned powercontrol circuit). The first comparator 414 may control the EMC shunt 413and the small second charge pump 407, which is may be turned on and offwhether the voltage limit at the emergency capacitor 105 is reached ornot.

The first comparator 414 compares an output voltage 415 of the voltagedivider 412, which is proportional to the voltage at the emergencycapacitor 105, with a first reference voltage 416 and provides amaximum-voltage-reached signal 417 on the basis of which the powercontrol circuit may activate and deactivate the second charge pump 407.The maximum-voltage-reached signal 417 may also be used to activate theEMC shunt 413 to clamp the voltage of the emergency capacitor 105 andtherefore to prevent the emergency capacitor 105 from overcharging. Themaximum-voltage-reached signal 417 may be fed into an OR-block 418together with the enable signal 322 to provide a stop-second-charge-pumpsignal 419 to the second charge pump 407. In other words, the powercontrol circuit may be configured to deactivate the second charge pump407, if the voltage of the antenna input signal 102 is equal to or belowthe given input threshold voltage and/or if the voltage at the emergencycapacitor 105 is in the predetermined range around (or below) themaximum voltage of the emergency capacitor 105.

In the first comparator 414, a hysteresis function may be implementedwhich means that the maximum-voltage-reached signal 417 is notimmediately reset after the voltage at the emergency capacitor 105 fallsbelow the maximum voltage, but (only) after the voltage at the emergencycapacitor 105 falls below the predetermined range.

In other words, the maximum-voltage-reached signal 417 signals that theemergency capacitor 105 has reached its maximum value or its targetvalue, and the second charge pump 407 is therefore deactivated. Thevoltage at the emergency capacitor 105 may drop within a certain rangewithout resetting the maximum-voltage-reached signal 417. This preventsuninterrupted on/off switching of the second charge pump 407.

The charge pump adjuster 406 may further be configured to provide thecharge pump adjust signal 405 based on the maximum-voltage-reachedsignal 417 provided by the first comparator 414. In other words, thecharge pump adjuster 406 may be configured to deactivate the firstcharge pump 402 if the emergency capacitor 105 is fully charged.

Additionally, as mentioned before, the digital control module 317 (orthe mentioned power control circuit) may further be configured toactivate the output stage 306 (for example, based on the enable signal322) and deactivate the first charge pump 402 and the second charge pump407, if the voltage of the antenna input signal 102, reaches or fallsbelow, the given input threshold voltage, and to deactivate the outputstage 306 (for example, using the enable signal 322) if the voltage ofthe antenna input signal 102 is above the given input threshold voltage.

After explaining the details of the charging circuit (or charge pumpcircuit) 304 and the emergency circuit 401, the output stage 306 will beexplained in more detail in the following. As mentioned before, thecharging circuit 304 is typically active as long as a magnetic field ispresent at the antenna 308, and therefore, as long as the voltage of theantenna input signal 102 is above the given input threshold voltage. Theoutput stage 306 is, in contrast to this, typically active during afield pause or a type A gap, and therefore if the voltage of the antennainput signal 102 is equal to or below the given input threshold voltage.

The step down converter circuit 207 of the output stage 306 may be aswitched capacitor step down converter circuit configured to provide astep down output voltage (for example, the voltage VDD) at the commonVDD node 312, based on a switching between a serial connection of atleast two step down capacitors 420 a, 420 b and a parallel connection ofthe at least two step down capacitors 420 a, 420 b. The use of aswitched capacitor step down circuit is an advantage because this may beimplemented using integrated circuit technologies without the need forinductive components.

The step down converter circuit 207 comprises a first converting stage421 and a second converting stage 422. As can be seen from FIG. 4, thefirst converting stage 421 and the second converting stage 422 may beswitched capacitor converting stages. The first converting stage 421 andthe second converting stage 422 are coupled in parallel in between theemergency capacitor 105 and the VDD node 312.

The output stage 306 further comprises a step down control logic 423(which, for example, may be comprised in an output control circuit). Thestep down control logic 423 is configured to generate a non-overlappingclock signal 424 controlling the first converting stage 421 and thesecond converting stage 422, such that in a first phase the contribution307 to the supply current 101 of the output stage 306 is provided at thecommon VDD node 312 by the first converting stage 421, and such that ina second state, the contribution 307 is provided at the common VDD node312 by the second converting stage 422. More precisely, the step downcontrol logic 423 generates a first push signal 425 and a first pullsignal 426 for controlling the first converting stage 421. Furthermore,the step down control logic 423 generates a second push signal 427 and asecond pull signal 428 for controlling the second converting stage 422.The push signals and the pull signals control switches of the convertingstages 421, 422. The first push signal 425 may be an inverse version ofthe second push signal 427 and the first pull signal 426 may be aninverse version of the second pull signal 428. In the above-mentionedfirst state, capacitors of the first converting stage 421 may be coupledin parallel to the VDD node 312 and capacitors of the second convertingstage 422 may be coupled in series with the emergency capacitor 105.Therefore, in the first state, charge is transferred from the capacitorsof the first converting stage 421 to the VDD node 312 and from theemergency capacitor 105 to the capacitors of the second converting stage422. In the second state this happens vice-versa. By implementing thetwo converting stages 421, 422, it can be ensured that the output stage406 can always (or almost uninterruptedly) deliver its contribution 307to the supply current 101. This is ensured by the alternating switchingof the first converting stage 421 and the second converting stage 422between the emergency capacitor 105 and the common VDD node 312.

As can be seen from FIG. 4, the first converting 421 stage may be afirst switched capacitor step down converter and the second convertingstage 422 may be a second switched capacitor step down converter. Thetwo converting stages 421, 422 may be equal, this means that they maycomprise the same number and sizes of capacitors and switches.

Furthermore, the step down control logic 423 may be configured tocompare a compare signal 429 derived from the supply voltage VDD at thecommon VDD node 312, with a first internal voltage 430 of the firstconverting stage 421 and a second internal voltage 431 of the secondconverting stage 422, to control a frequency and/or a duty cycle of thenon-overlapping clock signal 424 (and therefore frequencies and dutycycles of the push signals 425, 427 and the pull signals 426, 428) basedon the comparisons of the internal voltages 430, 431 with the comparesignal 429 derived from the supply voltage VDD, such that the supplyvoltage VDD at the common VDD node 312 is in a predetermined range.

The predetermined range for the supply voltage VDD may, for example, be1 to 2 V, 1.2 to 1.8 V, 1.3 to 1.5 V, or 1.4+/−5%.

The compare signal 429 may be a current or a voltage derived from thesupply voltage VDD and may be proportional to the supply voltage VDD torepresent the supply voltage VDD.

In other words, the step down control logic 423 is be configured tosense the internal voltages 430, 431 in the converting stages 421, 422(in the step down converters) and the supply voltage VDD at the commonVDD node 312 via the compare signal 429 to generate the non-overlappingclock signal 424 for the two converting stages 421, 422. The comparesignal 429 may be generated by a comparator outside the emergencycircuit 303.

The frequency and duty cycle of the non-overlapping clock signal 424controlling the two converting stages 421, 422 may be generated by anoscillator integrated in the step down control logic 423 and are adaptedto the present VDD voltage level.

As can be seen in FIG. 4, the output stage 306 may further comprise aswitch 440 and a second comparator 441. The switch 440 and the step downconverter circuit 207 (comprising the parallel connection of the firstconverting stage 421 and the second converting stage 422) are coupled inparallel between the emergency capacitor 105 and the common VDD node312. The step down control logic 423 of the output stage 306 isconfigured to open the switch 440 and activate the step down convertercircuit 207 (using the non-overlapping clock signal 424) if the voltageat the emergency capacitor 105 is above a given switched mode emergencycapacitor threshold voltage, such that the contribution 307 to thesupply current 101 provided at the VDD node 312 is based on a currentprovided by the step down converter circuit 207 (e.g. on a current fromthe first converting stage 421 or the second converting stage 422). Thestep down control logic 423 is further configured to close the switch440 and deactivate the step down converter circuit 207 (using thenon-overlapping clock signal 424) if the voltage at the emergencycapacitor 105 is equal to or below the given switched mode emergencycapacitor threshold voltage, such that the contribution 307 to thesupply current 101 provided at the VDD node 312 is a discharge current432 of the emergency capacitor 105 routed along the closed switch 440.

For providing this function within the step down control logic 423, thesecond comparator 441 may compare the voltage 415 which is proportionalto the voltage at the emergency capacitor 105 with a second referencevoltage 433. The second reference voltage 433 may be lower than thefirst reference voltage 416. Based on the comparison of the secondreference voltage 433 with the voltage 415 which is proportional to thevoltage at the emergency capacitor 105, the second comparator 441 maygenerate a trigger-linear-mode signal 434, which is fed into the stepdown control logic 423. The step down control logic 423 may sense thetrigger-linear-mode signal 434 and generate an enable-linear-switchsignal 435 and may also adjust the push signals 425, 427 and/or the pullsignals 426, 428 to the first converting stage 421 and the secondconverting stage 422, such that the first converting stage 421 and thesecond converting stage 422 are, optionally, disconnected from theemergency capacitor 105, if the switch 440 is closed.

The switch 440 may, for example, be a transistor, wherein theenable-linear-switch signal 435 may be coupled to a gate terminal of thetransistor or may, for example, be a switchable-linear-controller.

The output stage 306 can therefore work in two different modes: switchedcapacitor mode (if the first converting stage 421 and the secondconverting stage 422 are active or, in general, the step down convertercircuit 207 s active and the switch 440 is opened) or linear mode (ifthe (linear) switch 440 is closed and the step down converter circuit207 is deactivated. Switched capacitor mode is activated as long as thevoltage at the emergency capacitor 105 is above the given secondcapacitor threshold voltage where the two converting stages 421, 422 candeliver sufficient current, or, in other words, a sufficiently highcontribution 307 to the VDD node 312. If the voltage at the emergencycapacitor 105 is below the given switched-mode emergency capacitorthreshold voltage, the output stage 306 switches to linear mode in orderto deliver the current consumed at the VDD node 312. The decisionwhether switched capacitor mode or linear mode is activated is taken inthe step down control logic 423 of the output stage 306, based on thetrigger-linear-mode signal 434, generated by the second comparator 441.As mentioned before, the step down control logic 423 senses the internalvoltages 430, 431 in the first converting stage 421 and the secondconverting stage 422, and the voltage at the VDD node 312 via thecompare signal 429 derived from the supply voltage at the VDD node 312and generates the non-overlapping clock signal 424 for the twoconverting stages 421, 422. And furthermore, does the step down controllogic 423 generate the control signal 435 (the enable linear switchsignal 435) for the linear switch 440. The frequency and duty cycle ofthe non-overlapping clock signal 424 controlling the converting stages421, 422 are adapted to the present VDD voltage level and in the endadapted to the load current consumed from the VDD node 312.

In short, the high efficiency emergency circuit concept shown in FIG. 4results from the combination of an adaptive, multiple stage chargingcircuit 304 (comprising the first charge pump 402 and the second chargepump 407), the emergency capacitor 105 and the two stages, selfoscillating capacitive step down converting circuit 207 (comprising thefirst step down voltage converter 421, and the second step down voltageconverter 422). The first charge pump 422 and the second charge pump 407are turned on after discharge phase for recharging the emergencycapacitor 105 from a low voltage level. The second charge pump 407 isused to compensate current consumption and leakage current in theemergency capacitor module 401. During the discharge phase (indicated bythe discharge enable signal 322, derived from the pause signal 323 fromthe demodulator 314) the charging circuit 304 is stopped and the outputstage 306 (comprising the step down converter circuit 207) is activated.The contribution 307 to the supply current 101 is provided by the stepdown converter circuit 207 until the voltage at the emergency capacitor105 falls below the given switched mode emergency capacitor thresholdvoltage (as long as the step down converter circuit 207 can deliverenough current to the VDD node 312). Subsequently, if the voltage at theemergency capacitor 105 is equal to or below the switched-mode emergencycapacitor threshold voltage, the step down converter circuit 207 is

Some embodiments of the present invention provide, therefore, ahigh-efficiency emergency capacitor concept for transponder powersupplies allowing the reduction of the area of an emergency capacitor byat least a factor of 2.

FIG. 5 shows an example of a timing diagram of a single charge/dischargecycle as it may occur in the transponder power supply 300 from FIG. 3with the emergency circuit 303 from FIG. 4. The diagram shows, in afirst row, the voltage of the antenna input signal 102 as a function oftime. In a second row of the diagram, the discharge enable signal 322 asa function of time is shown. In a third row of the diagram, the chargepump adjust signal 405 as a function of time is shown. The value <000>for the charge pump adjust signal 405 corresponds in this concreteembodiment with the lowest charge current delivered from the firstcharge pump 402 and the value <111> corresponds with the highest chargecurrent delivered from the first charge pump 402. In a fourth row of thediagram, the supply voltage VDD at the common VDD node 312 as a functionof time, and the voltage VEMC at the emergency capacitor 105 as afunction of time are shown. In a fifth row of the diagram, themaximum-voltage-reached signal 417 as a function of time is shown. In asixth row of the diagram, the enable-linear-switch signal 435 as afunction of time is shown. In the diagram time is drawn at the x-axisand amplitudes (or logic states) of the different signals are drawn atthe y-axis.

In a first time interval 501, the voltage VLA/LB of the antenna inputsignal 102 is above the given input threshold voltage. In other words, amagnetic field is present at the antenna 308. Therefore, the chargingcircuit 304 is activated and the output stage 306 is deactivated. Thevoltage VEMC at the emergency capacitor 105 is increased and the voltageVDD at the common VDD node 312 is sourced from the regular power circuit310. From the third row of the diagram, it can be seen that the secondcharge pump 402 is started softly to not disturb the antenna inputsignal 102. The amount of charge transferred within one time unit fromthe antenna input signal 102 to the emergency capacitor 105 by the firstcharge pump 402 is increased with an increasing value of the charge pumpadjust signal 405. Therefore, at the end of the first time interval 501,the most charge per time unit is transferred from the antenna inputsignal 102 to the emergency capacitor 105. The emergency capacitor 105is charged to its maximum voltage. As mentioned before, the secondcharge pump 407 may also be switched on and transfer a small amount ofcharge (when compared to the amount of charge transferred by the firstcharge pump 402), from the VDD node 312 to the emergency capacitor 105.If the second charge pump 407 is switched on or off in this timeinterval 501 may be configurable and may be different for differentembodiments of the present invention. Therefore at least in someembodiments it can be sufficient if only the first charge pump 402 isswitched on during this time interval 501 to quickly charge theemergency capacitor 105.

At the end of the first time interval 501 the voltage at the emergencycapacitor 105 reaches the maximum voltage (i.e. the emergency capacitor105 is fully charged). Therefore in a subsequently following second timeinterval 502 of the diagram, the maximum-voltage-reached signal 417switches from logical “0” to logical “1”. Therefore, the first chargepump 402 and the second charge pump 407 are deactivated. As it can beseen from the diagram, a small amount of current is consumed by theemergency capacitor module 401 during the second time interval 502.

To compensate this in a subsequently following third time interval 503,the second charge pump 407 is activated, which transfers charge from theVDD node 312 to the emergency capacitor 105. This activation of thesecond charge pump 407 is triggered by the falling edge of the maximumvoltage reached signal 417. The second charge pump 407 is until theemergency capacitor 105 is fully charged, which is signaled by therising edge of the maximum-voltage-reached signal 417.

Therefore, in a subsequently following fourth time interval 504, theemergency capacitor 105 gets fully charged again.

In a subsequently following fifth time interval 505, a type A gap orfield pause occurs, i.e. the voltage VLA/LB of the antenna input signal102 drops to a voltage equal to or below the given input thresholdvoltage. The demodulator 414 senses this and sets the discharge enablesignal 322. An active charge pump (the first charge pump 402 and/or thesecond charge pump 407) would be deactivated and the output stage 306 isactivated in the switched capacitor mode. This means that the linearswitch 440 is opened and the first converting stage 421 and the secondconverting stage 422 of the step down converter circuit 207 areactivated (under the assumption that the voltage at the emergencycapacitor 105 is sufficiently high) and provide the contribution 307 tothe supply current 101. This can also be seen by the ripple at thesupply voltage VDD. Due to the fact that the first converting stage 421and the second converting stage 422 are sourced by the emergencycapacitor 105, the voltage VEMC at the emergency capacitor 105decreases, and the maximum-voltage-reached signal 417 is reset. At theend of the fifth time interval 505, the comparator 441 detects that thevoltage VEMC falls below the given switched-mode emergency capacitorthreshold voltage and signals this via the trigger-linear-mode signal434 to the step down control logic 423, which then closes the linearswitch 440 by setting the enable linear switch signal 435 and bydeactivating the step down converter circuit 207.

In a subsequently following sixth time interval 506, the output stage306 is therefore in the linear mode, wherein the contribution 307 to thesupply current 101 provided at the VDD node 312 is equivalent to thedischarge current 432 of the emergency capacitor 105 routed along thelinear switch 440.

At the beginning of the subsequently following seventh time interval507, the type A gap or field pause is over and the voltage VLA/LB of theantenna input signal 102 increases above the given input thresholdvoltage and the demodulator 314 detects the end of the type A gap orfield pause and therefore the discharge enable signal 322 is reset bythe digital control module 317, deactivating the output stage 306.Furthermore, the first charge pump 402 and the second charge pump 407are activated again and the emergency capacitor 105 is recharged quicklyto its maximum voltage.

In the embodiments described above, it has been assumed that thedemodulator 314 sets the power signal 323 if the voltage of the antennainput signal 102 is equal to or below a given input threshold voltageand resets the power signal if the voltage of the antenna input signal102 is above the given threshold input voltage. According to furtherembodiments, the demodulator 314 may comprise a hysteresis function,which means the demodulator 314 may be configured to set the pausesignal 323 if the voltage of the antenna signal 102 is equal to or belowa given first input threshold voltage and to reset the power signal 323if the voltage of the antenna input signal 102 is above a given secondthreshold voltage. The given first input threshold voltage may bedifferent from the given second input threshold voltage.

FIG. 6 shows a block diagram of a transponder 600 according to anembodiment of the present invention. The transponder 600 comprises anantenna circuit 601, a transponder power supply 602 and a receivercircuit 603. The antenna circuit 601 may, for example, be equal to theantenna 308 with the external tuning capacitor 309 shown in FIG. 3. Thetransponder power supply 602 may, for example, be equal to thetransponder power supply 100 from FIG. 1, the transponder power supply200 from FIG. 2, or the transponder power supply 300 from FIG. 3. Thetransponder power supply 602 is configured to provide a supply current101 based on an antenna input signal 102. The transponder power supply602 provides the supply current 101 to the receiver circuit 603. Thereceiver circuit 603, which is sourced from the transponder power supply602, may, for example, be configured to demodulate an amplitudemodulated signal, for example, according to the ISO 14443 A Standard.Instead of a receiver circuit 603, the trans-ponder 600 may alsocomprise a transceiver circuit which may also be configured to receivean amplitude modulated signal. The transceiver circuit may further beconfigured to transmit data using load modulation schemes.

FIG. 7 shows a flow diagram of a method 700 for providing a supplycurrent based on an antenna input signal. The method 700 comprises astep 701 of charging an emergency capacitor based on the antenna inputsignal to a maximum voltage which is higher than a voltage of theantenna input signal. Furthermore, the method 700 comprises a step 702of providing a contribution to the transponder supply current using acharge of the emergency capacitor. The step 701 may for examplecorrespond to the time interval 501 of the diagram shown in FIG. 5, andthe step 702 may for example correspond to the time intervals 505 and506 of the diagram shown in FIG. 5. The method 700 may, for example, beperformed using the transponder power supply 100 shown in FIG. 1.

FIG. 8 shows a flow diagram of a method 800 for providing a transpondersupply current based on an antenna input signal. The method 800comprises a step 801 of charging an emergency capacitor based on anantenna input signal. Furthermore, the method 800 comprises a step ofperforming a step down conversion on the basis of a charge of theemergency capacitor to provide a contribution to the transponder supplycurrent. The step 801 may, for example, correspond to the time interval501 of the diagram shown in FIG. 5 and the step 802 may, for example,correspond to the time interval 505 of the diagram shown in FIG. 5. Themethod 800 may, for example, be performed by the transponder powersupply 200 according to FIG. 2.

The methods 700, 800 may be supplemented by any of the features andfunctionalities described herein with respect to the apparatus, and maybe implemented using the hardware components of the apparatus.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper” and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

What is claimed is:
 1. A transponder power supply for providing a supplycurrent based on an antenna input signal, the transponder power supplycomprising: an emergency circuit comprising a charging circuit, anemergency capacitor and an output stage; wherein the charging circuit isconfigured to charge the emergency capacitor based on the antenna inputsignal to a maximum voltage, which is higher than a voltage of theantenna input signal; and wherein the output stage is configured toprovide a contribution to the supply current using a charge of theemergency capacitor.
 2. The transponder power supply according to claim1, wherein the output stage comprises a step down converter circuitconfigured to provide the contribution to the supply current using thecharge of the emergency capacitor.
 3. The transponder power supplyaccording to claim 1, wherein the charging circuit comprises a firstcharge pump coupled to an antenna port for the antenna input signal; andwherein the first charge pump is configured to provide a first chargecurrent to the emergency capacitor based on the antenna input signal. 4.The transponder power supply according to claim 3, wherein the firstcharge pump is configured to vary an amount of charge transferred fromthe antenna input signal to the emergency capacitor in dependence on acharge-pump-adjust signal; and wherein the transponder power supplycomprises a charge pump adjuster configured to provide thecharge-pump-adjust signal in dependence on a voltage of the antennainput signal, such that a first amount of charge transferred within agiven time unit in a first state, in which an amplitude of the voltageof the antenna input signal is comparatively small, is smaller than asecond amount of charge transferred within the given time unit in asecond state, in which the amplitude of the voltage of the antenna inputsignal is comparatively larger.
 5. The transponder power supplyaccording to claim 1, comprising a regular power circuit coupled inparallel to the emergency circuit, the regular power circuit beingconfigured to provide another contribution to the supply current at acommon VDD node, if the voltage of the antenna input signal is above agiven input threshold voltage; and wherein the emergency circuit isconfigured to selectively provide its contribution to the supply currentat the common VDD node, if the voltage of the antenna input signal isequal to, or below, the given input threshold voltage.
 6. Thetransponder power supply according to claim 5, wherein the chargingcircuit comprises a first charge pump coupled to an antenna port for theantenna input signal; wherein the first charge pump is configured toselectively provide a first charge current to the emergency capacitor bytransferring charge from the antenna port, if the voltage of the antennainput signal is above the given threshold input voltage; and wherein thecharging circuit comprises a second charge pump configured toselectively provide a second charge current to the emergency capacitorby transferring charge from the common VDD node, if the voltage of theantenna input signal is above the given threshold input voltage.
 7. Thetransponder power supply according to claim 6, wherein the second chargepump is configured to provide the second charge current until theemergency capacitor reaches its maximum voltage, to interrupt providingthe second charge current until a voltage at the emergency capacitorfalls below a given leakage current emergency capacitor thresholdvoltage, and to resume providing the second charge current until theemergency capacitor reaches its maximum voltage.
 8. The transponderpower supply according to claim 6, wherein an output of the first chargepump and an output of the second charge pump are coupled to theemergency capacitor, such that an overall charge current of theemergency capacitor is a sum of the first charge current and the secondcharge current.
 9. The transponder power supply according to claim 6,wherein the first charge pump and the second charge pump are configuredsuch that a maximum charge current provided by the first charge pump is,at least by a factor of 10, bigger than a maximum charge currentprovided by the second charge pump.
 10. The transponder power supplyaccording to claim 1, further comprising a power control circuitconfigured to activate and deactivate the charging circuit in dependenceon the voltage of the antenna input signal and a voltage at theemergency capacitor.
 11. The transponder power supply according to claim10, wherein the charging circuit comprises a first charge pump and asecond charge pump; and wherein the power control circuit is configuredto activate both the first charge pump and the second charge pump afteran increase of the voltage of the antenna input signal from a voltagebelow a given input threshold voltage to a voltage above the given inputthreshold voltage in order to charge the emergency capacitor to themaximum voltage, to subsequently deactivate both the first charge pumpand the second charge pump, and to keep the voltage at the emergencycapacitor within a predetermined range using the second charge pump,while the voltage of the antenna input signal is above the given inputthreshold voltage.
 12. The transponder power supply according to claim11, wherein the power control circuit is further configured to activatethe output stage and deactivate the first charge pump and the secondcharge pump if the voltage of the antenna input signal reaches or fallsbelow the given input threshold voltage and to deactivate the outputstage and activate the first charge pump and the second charge pump ifthe antenna input signal is above the given input threshold voltage. 13.The transponder power supply according to claim 1, wherein the outputstage comprises a switch, a voltage converter circuit and an outputcontrol circuit; wherein the switch and the voltage converter circuitare coupled in parallel between the emergency capacitor and a common VDDnode; and wherein the power control circuit is configured to open theswitch and activate the voltage converter circuit if a voltage at theemergency capacitor is above a given switched-mode emergency capacitorthreshold voltage, such that the contribution to the supply currentprovided at the common VDD node is based on a current provided by thevoltage converter circuit, and to close the switch and deactivate thevoltage converter circuit if the voltage at the emergency capacitor isequal to, or below, the given switched-mode emergency capacitorthreshold voltage, such that the contribution to the supply currentprovided at the common VDD node is based on a discharge current of theemergency capacitor routed along the closed switch.
 14. A transponderincluding the transponder power supply according to claim 1, thetrans-ponder further including an antenna and a receiver circuitconfigured to receive an amplitude modulated datastream within theantenna input signal, the transponder power supply being configured toprovide the supply current to the receiver circuit based on the antennainput signal from the antenna.
 15. The transponder according to claim14, wherein the receiver circuit is configured to communicate with atransponder reader complying with the ISO 14443 A standard.
 16. Atransponder power supply for providing a supply current based on anantenna input signal, the transponder power supply comprising: anemergency circuit comprising a charging circuit, an emergency capacitor,and an output stage; wherein the charging circuit is configured tocharge the emergency capacitor based on the antenna input signal; andwherein the output stage comprises a step down converter circuitconfigured to provide a contribution to the supply current using acharge of the emergency capacitor.
 17. The transponder power supplyaccording to claim 16, wherein the step down converter circuit is aswitched capacitor step down converter circuit configured to provide astep down output voltage at a common VDD node based on a switchingbetween a serial connection of at least two step down capacitors and aparallel connection of the at least two step down capacitors.
 18. Thetransponder power supply according to claim 16, wherein the output stagecomprises an output control circuit, wherein the step down convertercircuit comprises a first converting stage and a second converting stagecoupled in parallel between the emergency capacitor and a common VDDnode; and wherein the output control circuit is configured to generate anon-overlapping clock signal controlling the first converting stage andthe second converting stage, such that in a first state the contributionto the supply current is provided at the common VDD node by the firstconverting stage, and such that in a second state the contribution tothe supply current is provided at the common VDD node by the secondconverting stage.
 19. The transponder power supply according to claim18, wherein the output control circuit is configured to compare a signalderived from a supply voltage at the common VDD node with an internalvoltage of the first converting stage and an internal voltage of thesecond converting stage, and to control a frequency or a duty cycle ofthe non-overlapping clock signal based on the comparisons of theinternal voltages with the signal derived from the supply voltage, suchthat a supply voltage VDD at the common VDD node is in a predeterminedrange.
 20. The transponder power supply according to claim 16, whereinthe output stage comprises a switch and an output control circuit;wherein the switch and the step down converter circuit are coupled inparallel between the emergency capacitor and a common VDD node; andwherein the output control circuit is configured to open the switch andactivate the step down converter circuit if the voltage at the emergencycapacitor is above a given switched-mode emergency capacitor thresholdvoltage, such that the contribution to the supply current provided atthe common VDD node is based on a current provided by the step downconverter circuit, and to close the switch and deactivate the step downconverter circuit if the voltage at the emergency capacitor is equal to,or below, the given switched-mode emergency capacitor threshold voltage,such that the contribution to the supply current provided at the commonVDD node is a discharge current of the emergency capacitor routed alongthe closed switch.
 21. The transponder power supply according to claim16, further comprising a power control circuit configured to activatethe charging circuit and deactivate the output stage if a voltage of theantenna input signal is above a given input threshold voltage and todeactivate the charging circuit and activate the output stage if thevoltage of the antenna input signal is equal to, or below, the giveninput threshold voltage.
 22. A transponder including the transponderpower supply according to claim 16, the transponder further including anantenna and a receiver circuit configured to receive an amplitudemodulated datastream within the antenna input signal, the transponderpower supply being configured to provide the supply current to thereceiver circuit based on the antenna input signal from the antenna. 23.The transponder according to claim 22, wherein the receiver circuit isconfigured to communicate with a transponder reader complying with theISO 14443 A standard.
 24. A method for providing a transponder supplycurrent based on an antenna input signal, the method comprising:charging an emergency capacitor based on the antenna input signal to amaximum voltage, which is higher than a voltage of the antenna inputsignal; and providing a contribution to the transponder supply currentusing a charge of the emergency capacitor.
 25. A method for providing atransponder supply current based on an antenna input signal, the methodcomprising: charging an emergency capacitor based on the antenna inputsignal; and performing a step down conversion on the basis of a chargeof the emergency capacitor to provide a contribution to the transpondersupply current.
 26. A transponder power supply for providing a supplycurrent based on an antenna input signal, the transponder power supplycomprising: an emergency circuit comprising a charging circuit, anemergency capacitor, a power control circuit, and an output stage;wherein the charging circuit comprises a first charge pump and a secondcharge pump; wherein the first charge pump and the second charge pumpare configured to charge the emergency capacitor based on the antennainput signal to a maximum voltage, which is higher than a voltage of theantenna input signal; wherein an output of the first charge pump and anoutput of the second charge pump are coupled to the emergency capacitor,such that an overall charge current of the emergency capacitor is a sumof a first charge current of the first charge pump and of a secondcharge current of the second pump; wherein a maximum charge currentprovided by the first charge pump is at least by a factor of 10 largerthan a maximum charge current provided by the second charge pump;wherein the power control circuit is configured to activate the chargingcircuit and deactivate the output stage if the voltage of the antennainput signal is above a given input threshold voltage and to deactivatethe charging circuit and activate the output stage if the voltage of theantenna input signal is equal to, or below, the given input thresholdvoltage; wherein the power control circuit is configured to activateboth, the first charge pump and the second charge pump after an increaseof the antenna input signal from a voltage below the given inputthreshold voltage to a voltage above the given input threshold voltage,in order to charge the emergency capacitor to the maximum voltage, tosubsequently deactivate both, the first charge pump and the secondcharge pump, and to keep the voltage at the emergency capacitor within apredetermined range using the second charge pump, while the voltage ofthe antenna input signal is above the given input threshold voltage;wherein the first charge pump is configured to vary an amount of chargetransferred from the antenna input signal to the emergency capacitor independence on a charge pump adjust signal; wherein the second chargepump is configured to provide the second charge current by transferringcharge from a common VDD node at which the contribution to the supplycurrent is provided; wherein the output stage comprises a switchedcapacitor step down converting circuit, a switch and an output controlcircuit; wherein the switched capacitor step down converting circuitcomprises a first converting stage and a second converting stage;wherein the first converting stage, the second converting stage and theswitch are coupled in parallel between the emergency capacitor and thecommon VDD node; wherein the output control circuit is configured toopen the switch, to activate the first and the second converting stageand to provide a non-overlapping clock signal to the first convertingstage and the second converting stage, if a voltage at the emergencycapacitor is above a given switched mode emergency capacitor thresholdvoltage, such that the contribution to the supply current provided atthe common VDD node is based on a current provided switchably either bythe first converting stage or the second converting stage and to closethe switch and to deactivate the first converting stage and the secondconverting stage, if the voltage at the emergency capacitor is equal to,or below, the given switched mode emergency capacitor threshold voltage,such that the contribution to the supply current provided at the commonVDD node is a discharge current of the emergency capacitor routed alongthe closed switch.